\[\begin{array}{l}
a)\frac{{15}}{{17}};\frac{{29}}{{32}}\\
MCS:544\\
= \frac{{15 \times 32}}{{17 \times 32}} = \frac{{480}}{{544}}\\
\frac{{29}}{{32}} = \frac{{29 \times 17}}{{32 \times 17}} = \frac{{493}}{{544}}\\
b)\frac{{12}}{{18}} = \frac{{12:6}}{{18:6}} = \frac{2}{3}\\
\frac{{13}}{{17}}\\
MSC:51\\
\frac{2}{3} = \frac{{2 \times 17}}{{3 \times 17}} = \frac{{34}}{{51}}\\
\frac{{13}}{{17}} = \frac{{13 \times 3}}{{17 \times 3}} = \frac{{39}}{{51}}\\
c)\frac{{16}}{{51}};\frac{{31}}{{90}}\\
MSC:1530\\
\frac{{16}}{{51}} = \frac{{16 \times 30}}{{51 \times 30}} = \frac{{480}}{{1530}}\\
\frac{{31}}{{90}} = \frac{{31 \times 17}}{{90 \times 17}} = \frac{{527}}{{1530}}\\
d)\frac{{21}}{{25}};\frac{{600}}{{81}}\\
\frac{{600}}{{81}} = \frac{{600:3}}{{81:3}} = \frac{{200}}{{27}}\\
MSC:675\\
\frac{{21}}{{25}} = \frac{{21 \times 27}}{{25 \times 27}} = \frac{{567}}{{675}}\\
\frac{{200}}{{27}} = \frac{{200 \times 25}}{{27 \times 25}} = \frac{{5000}}{{675}}
\end{array}\]